In British Patent Application No 2 306 916 A1 there is proposed a device, the prior art device, for transporting the ribbon in a thermal transfer or other printer past the print head in which a shuttle mechanism acts upon the ribbon on either side of the print head. Due to the fact that the shuttle draws a double sided loop of material with it as it reciprocates, the ribbon is transported past the print head at twice the speed of the shuttle. Such a mechanism is thus of especial application in printing onto substrates which are moving rapidly past the print head. A conventional printer ribbon transport mechanism using driven ribbon supply and take up reels cannot achieve fast transport of the ribbon in accurate increments due to the inertia of the reels. The rapid start and stop action of such a conventional feed mechanism may also apply excessive stresses on the ribbon leading to breakage of the ribbon. The use of a shuttle mechanism is claimed to avoid such stresses.
The use of a shuttle mechanism is also claimed to achieve accurate registration of the printer ribbon with the print head since the shuttle achieves accurate movement of the ribbon in known increments past the print head. In the preferred embodiment, the shuttle mechanism is driven by a belt loop carrying the shuttle secured to the belt and travelling upon linear guide rails. The belt is moved in one direction or the other by means of a stepper motor so that the belt and hence the shuttle moves a specified and fixed distance at each movement of the belt. However, such a shuttle mechanism is complex to construct and operate and requires that the shuttle be repeatedly accelerated and decelerated at each end of its reciprocal travel. Furthermore, it is necessary to ensure accurate linear travel of the shuttle by means of guide means, which require to be manufactured to a high degree of accuracy if they are not to be prone to jam. This is particularly the case where the print head, which must accurately depress the ribbon into contact with the moving substrate, is carried on the shuttle mechanism. This adds to the cost of manufacture and maintenance of the shuttle mechanism.
We have now devised a device which reduces the complexity of the prior art device. In the device of the invention, the linearly moving shuttle of the prior art device is replaced by a rocking beam mechanism. This can be operated without the need for guides to guide its movement using a single stepper motor resulting in a simpler mechanism which is less prone to jam. The amount of ribbon transported past the print head can readily be varied by simply varying the angle through which the beam rocks. Furthermore, the speed and/or the amount of ribbon transported can also be varied simply by varying the effective length of the beam and/or by providing additional pairs of ribbon guides on the rocking beam so that two or more loops of ribbon are drawn on each side of the print head as described below. In the device of the prior art, it is necessary to vary the length of travel of the shuttle in order to vary the amount of ribbon transported past the print head. Whilst the length of travel, or throw, of each stroke of the shuttle mechanism can be increased or decreased by varying the number of rotations of the stepper motor driving the drive belt which carries the shuttle, it is not possible to increase the speed at which that increased amount of ribbon travels past the print head. As a result, the prior art device cannot readily accommodate changes in the length of the message printed on the substrate (and hence the length of ribbon to be carried past the print head) without slowing the speed at which the substrate travels past the print head. The use of a rocking beam mechanism enables the speed of transport of the ribbon past the print head to be increased at a given stepper motor speed simply by increasing the effective length of the beam.
The use of a rocking beam mechanism thus provides a number of advantages over the device of the prior art and gives the operator a mechanism whose ribbon transport can be readily adjusted by simple adjustment of the effective length of the beam.